Composite sandwich structures may be used in a variety of applications because of their favorable strength-to-weight ratios. For example, such sandwich structures may be used, without limitation, in aerospace applications as lightweight structural panels. A typical composite sandwich used in the aerospace industry comprises a lightweight, high strength core sandwiched between a pair of composite facesheets, each formed from laminated plies of fiber reinforced polymer resin. In some cases, surface unevenness at the interface between the facesheets and the core may result in undesired properties in the cured structure due to less than desired compaction pressure applied to the facesheet plies during curing. In order to cause the interface surfaces of the facesheets and the core to conform to each other, the sandwich structure may be cured in an autoclave which applies sufficient pressure to the sandwich structure during curing to compact the facesheets and force the core and the facesheets to conform to each other. Alternatively, irregular features and/or surface unevenness on the facesheets may be substantially reduced by fully curing the facesheets prior to being assembled with the core in a secondary bonding operation. Still another solution to the problem involves machining the core to substantially match the irregular features or uneven surfaces of the facesheets.
Each of the solutions mentioned above may have disadvantages. Processing sandwich structures in autoclaves may be relatively expensive in terms of capital costs, and may be labor intensive because of the procedures required to prepare the sandwich structure for autoclave processing. Fully curing each of the facesheets prior to bonding them to the core may also be unsatisfactory because the core may have difficulty conforming to irregular features on facesheet surfaces that have hardened and become less compliant as a result of curing. Finally, machining the core surfaces to match those of the facesheets may be time consuming and labor intensive.
Accordingly, there is a need for a method of making composite sandwich structures that may be performed outside of an autoclave, and which allows co-curing of the facesheets with the core while minimizing porosities.